Functions using enthalpy or entropy as an independent variable

This topic explains how steam property functions behave when enthalpy or entropy is used as an input, with a focus on the wet steam region. It also summarizes limitations in how the ASME routines handle edge cases and out‑of‑range inputs.

Note: While the descriptions align with current ASME/IAPWS standards and internal documentation, users should verify function behavior against the latest product implementation and documentation, especially when modeling near-critical conditions or using extrapolated values.

Valid input ranges

According to ASME standards:

• Enthalpy: −1 to 1860 BTU/lbm

• Entropy: −0.1 to 3.0 BTU/lbm·°R

However, some combinations of pressure and enthalpy/entropy correspond to compressed water, not steam. These inputs result in an error state.

Behavior in the wet steam region

Steam property functions behave differently depending on how enthalpy or entropy inputs relate to saturated liquid and vapor boundaries. The following summarizes how each function handles these conditions:

• VPH and VPS

  • Use the Clausius-Clapeyron equation and saturated vapor values.

  • Computed volume may differ slightly from values derived using vapor fraction.

  • Differences increase with higher moisture content.

  • No modification is made to the ASME equation since steam with vapor fraction > 0.5 is typical in practice.

• HPS and SPH

  • Use the equation: H_vap = T_s · S_vap and rely on saturated vapor values.

  • Do not check if input enthalpy/entropy is below saturated liquid values.

  • Return extrapolated values instead of errors.

• TPH and TPS

  • Check if input enthalpy/entropy is less than saturated vapor values. If so, they return saturated temperature.

  • Do not handle inputs below saturated liquid values—this causes error states.

• XPH and XPS

  • Do not validate input bounds.

  • If input is greater than saturated vapor values, they return 0.

  • If input is less than saturated liquid values, they return negative vapor fraction instead of an error.

Summary of limitations

ASME routines lack robust validation for inputs outside the saturated liquid/vapor range. This can lead to:

• Extrapolated outputs instead of warnings

• Incorrect vapor fractions

• Unexpected error states

Behavior above the critical point

For pressures above the critical point (3200.112 psia), the VPH, VPS, HPS, SPH, TPH, and TPS functions can compute valid results for states where the temperature is between 682°F and the critical temperature of 705.10°F. Although these states are technically considered compressed water, not steam, the functions still return usable outputs.

This behavior may be relevant for advanced modeling scenarios but should be used with caution, as it falls outside typical steam property boundaries.

Practical considerations

Although the ASME routines have inadequate input validation in the wet steam region, they are still widely used in practice due to their consistency and alignment with typical steam conditions.